KATP channel Kir6.2 E23K variant overrepresented in human heart failure is associated with impaired exercise stress response

Abstract

ATP-sensitive K+ (K(ATP)) channels maintain cardiac homeostasis under stress, as revealed by murine gene knockout models of the KCNJ11-encoded Kir6.2 pore. However, the translational significance of K(ATP) channels in human cardiac physiology remains largely unknown. Here, the frequency of the minor K23 allele of the common functional Kir6.2 E23K polymorphism was found overrepresented in 115 subjects with congestive heart failure compared to 2,031 community-based controls (69 vs. 56%, P < 0.001). Moreover, the KK genotype, present in 18% of heart failure patients, was associated with abnormal cardiopulmonary exercise stress testing. In spite of similar baseline heart rates at rest among genotypic subgroups (EE: 72.2 ± 2.3, EK: 75.0 ± 1.8 and KK:77.1 ± 3.0 bpm), subjects with the KK genotype had a significantly reduced heart rate increase at matched workload (EE: 32.8 ± 2.7%, EK: 28.8 ± 2.1%, KK: 21.7 ± 2.6%, P < 0.05), at 75% of maximum oxygen consumption (EE: 53.9 ± 3.9%, EK: 49.9 ± 3.1%, KK: 36.8 ± 5.3%, P < 0.05), and at peak V(O2) (EE: 82.8 ± 6.0%, EK: 80.5 ± 4.7%, KK: 59.7 ± 8.1%, P < 0.05). Molecular modeling of the tetrameric Kir6.2 pore structure revealed the E23 residue within the functionally relevant intracellular slide helix region. Substitution of the wild-type E residue with an oppositely charged, bulkier K residue would potentially result in a significant structural rearrangement and disrupted interactions with neighboring Kir6.2 subunits, providing a basis for altered high-fidelity K(ATP) channel gating, particularly in the homozygous state. Blunted heart rate response during exercise is a risk factor for mortality in patients with heart failure, establishing the clinical relevance of Kir6.2 E23K as a biomarker for impaired stress performance and underscoring the essential role of K(ATP) channels in human cardiac physiology.

Fig. 1

K23 variant of Kir6.2 is overrepresented in a heart failure cohort. Restriction enzyme digestion of PCR-amplified genomic DNA allowed determination of Kir6.2 E23K genotypes. a The c.67G→A single nucleotide polymorphism within codon 23, coding for the K allele, resulted in abolition of a BanII restriction site and discrete fragment sizes for homo- (EE and KK) and hetero-zygous (EK) DNA samples. b Compared to frequencies of EE, KK and EK genotypes in the ethnically matched population of Olmsted County (Rochester, MN), the presence of the K23 allele is overrepresented in a cohort of patients with heart failure

Fig. 2

Demographic information and left ventricular structure and function. Data are shown as mean ± SD, or count (%) where appropriate. No significant statistical differences among the three genotypes were found in the study cohort at baseline. Weight and body mass index (BMI), blood pressure (BP), left ventricular (LV) morphology and contractile function, as well as blood levels of catechola-mines (norepinephrine) and B-type natriuretic peptide (BNP), were similar among groups. The proportion of subjects presenting with ischemic heart disease or treated with β-blockers was equivalent across genotype groups

Fig. 3

KK genotype is associated with abnormal response to exercise stress. Heart failure subjects underwent a progressive exercise stress test, starting at a warm-up matched workload level followed by increasing workloads at 2-min intervals until subjects could no longer exercise. Variables were analyzed post hoc at the initial matched work-load level, at peak VO₂ and at an intermediate level 75% of peak. a In spite of comparable heart rates at rest (EE: 72.2 ± 2.3, EK: 75.0 ± 1.8 and KK: 77.1 ± 3.0 beats per minute; P = 0.39), b individuals with the KK genotype demonstrated an abnormal heart rate response at all levels of the cardiopulmonary exercise stress test (EE: 32.8 ± 2.7%, EK: 28.8 ± 2.1%, KK: 21.7 ± 2.6% at the initial matched workload level, EE: 53.9 ± 3.9%, EK: 49.9 ± 3.1%, KK: 36.8 ± 5.3% at 75% of peak VO₂, and EE: 82.8 ± 6.0%, EK: 80.5 ± 4.7%, KK: 59.7 ± 8.1% at peak VO₂, * P < 0.05)

Fig. 4

Cardiac function during cardiopulmonary exercise stress test. Parameters of cardiovascular and respiratory function were obtained at rest, at a first stage of matched workload for all individuals (Matched), at 75% of peak oxygen consumption (75%), and at peak oxygen consumption (peak) on a treadmill exercise stress test. Amount of work and duration of exercise were equivalent in all groups throughout the exercise stress test, along with similar respiratory exchange ratio (RER), oxygen pulse (O₂ pulse), ventilatory equivalent ratio for carbon dioxide (VE/VCO₂), rate pressure product and oxygen uptake efficiency slope (OUES)

Fig. 5

Molecular model of Kir6.2 channel pore. a Sequence alignment of mammalian KCNJ11-encoded Kir6.2 shows highly conserved amino acid residues in N-terminus. The predicted secondary structure below the alignment marks was obtained from PredictProtein (http://www.predictprotein.org). b Ribbon illustration of Kir6.2 representing individual subunits. A slide helix, involved in lateral movement for the gating mechanism, lies between the transmembrane (TM) and intracellular (IC) domains. The dimensions of TM and IC domains are ~44 and ~72 Å long, respectively. E23 is denoted. c A tetramer of Kir6.2 viewed from above with a partial removal of TM and IC domains to clarify the location of the slide helix, E23, and the cytosolic pore which pervades the IC domains. d Structure of two Kir6.2 subunits with TM α-helices, slide helix, and the previously unrecognized first α-helix region. The E23 reside is located at the inference between TM and IC domains, where a cluster of charged residues (negative E19/ D20/D58/D65 and positive R16/R54/K67) interacts with phospholipid head groups. Substitution of K23 could be involved in electrostatic repulsion with R325 from an adjacent subunit